The 'Spanish' influenza pandemic of 1918-19 was the largest outbreak of any infectious disease in recorded history, killing approximately 20-40 million people worldwide. Direct study of this virus has been impossible because the pandemic pre-dated the first successful identification and isolation of the causative agent. However, recent gene assembly of viral RNA fragments has facilitated insights into the virus composition. The main goal of this proposal is to study the protein antigens from these assembled genes to understand, from a structural viewpoint, why this virus was so virulent. Protein components of the virus will be cloned, expressed and purified in sufficient quantities to crystallize and determine their X-ray structures. Initially, the main thrust of this project will be to study the two major antigenic epitopes, hemagglutinin (HA) and neuraminidase (NA). The structures of both proteins will be determined alone and in complex with relevant ligands. For the hemagglutinin, both the precursor (HA0) and cleaved protein (HAl/HA2) will be investigated. Fragments of the HA will be produced to elucidate the acid-induced conformational changes which occur during virus entry and membrane fusion. In addition, complexes with neutralizing antibodies will investigate the structural basis of viral neutralization. For the neuraminidase, complexes with substrate and current antiviral drugs, effective against influenza viruses will aid in future design of anti-virals specific for the Spanish flu. Finally, a third 1918 influenza protein, the multifunctional nonstructural protein (NS1) will be investigated. These structural results can be combined with data from the other projects to explain the severity of this virus and what steps can be taken to counter future outbreaks, originating from either natural or bio-terrorist sources.